Fast string operations, Was x86 CPU bug in rep movsb

UPDATE:  This isn’t a bug after all.  Aspect provided documtation of what is actually occuring.

It’s a feature since pentium pro computers to do ‘fast string’ or block operations.  A block operation (eg, movb) of 64 bytes is performeed if ecx >= 64, if edi is aligned to 8 an byte boundary, and if esi and edi are not both in the same cachline (64 byte block).  Otherwise, it performs a single operations.

This seems to have resolved my emulation problems 🙂

While unpacking MEW in my emulator, I came across an interesting bug.  single stepping through rep movsb with ecx=65 completes the instruction in 2 steps.

movsb copies a byte from the memory pointed to by esi, into the memory pointed to be edi.  the rep part of the instruction, repeats the movsb ecx times.  It does this by iteratively decrementing the ecx register until it is 0.

On my computer, an old P4, single stepping rep movsb with ecx 65,  single steps from ecx=65  to ecx=1.  This is incorrect (I presume), it should single step through every decrement of ecx.

nemo courteously tested this bug on his own PC, and reported that it single stepped through every decrement of ecx.  This bug is probably specific to my CPU type.


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